Direct involvement of the small GTP-binding protein Rho in lbc oncogene function

The lbc oncogene is tumorigenic in nude mice, transforms NIH 3T3 fibroblasts, and encodes a Dbl homology domain found in several transforming gene products including the dbl oncogene product. While both lbc- and dbl-transformed NIH 3T3 foci exhibited a comparable gross appearance, lbc-transformed cell morphology was clearly distinct from that of dbl-transformed cells. Given these differences, we investigated the biochemical activity and target specificity of the Lbc oncoprotein both in vivo and in vitro. Here we show that Lbc associates specifically with the GTP-binding protein Rho in vivo, but not with the Ras, Rac, or Cdc42Hs GTP-binding proteins, and that recombinant, affinity-purified Lbc specifically catalyzes the guanine-nucleotide exchange activity of Rho in vitro. Consistent with an in vivo role for Lbc in Rho regulation, we further demonstrate that micro-injected onco-lbc potently induces actin stress fiber formation in quiescent Swiss 3T3 fibroblasts indistinguishable from that induced by Rho. Finally, lbc-induced NIH 3T3 focus formation is inhibited by co-transfection with a rho dominant-negative mutant. These results strongly indicate that the lbc oncogene encodes a specific guanine nucleotide exchange factor for Rho and causes cellular transformation through activation of the Rho signaling pathway.